1. Field of the Invention
The present invention relates to an oscillating circuit having a function for outputting an oscillating signal by employing a crystal oscillating element or the like, or for outputting an oscillating signal in response to a clock signal supplied from outside.
2. Description of Related Art
A conventional crystal oscillating circuit has a crystal oscillator, a resisting means, a NOR gate, and inverter. One electrode of the crystal oscillator is coupled to a first terminal via a switch, while the other electrode of the crystal oscillator is coupled to a second terminal. The resisting means is coupled between the first terminal and the second terminal. The resisting means is composed of a p-channel type field-effect transistor (hereinafter referred to as "PMOS") and an n-channel field-effect transistor (hereinafter referred to as "NMOS"), the PMOS and the NMOS being connected in parallel to each other between the first terminal and the second terminal. The gate of the PMOS is connected to a ground GND to which a ground potential is applied, while the gate of the NMOS is connected to a power source VDD to which a power potential is applied. The PMOS and the NMOS are always in a conducting state. One input terminal of the NOR gate is connected to the first terminal, the NOR gate being connected between the power source VDD and the ground GND. A standby signal STBY is applied to the other input terminal of the NOR gate. The output terminal of the NOR gate is connected to the second terminal. The inverter is connected to the output terminal of the NOR gate.
The operation of the conventional oscillating circuit will now be described. The standby signal STBY is a signal for setting a low power consumption mode. In the low power consumption mode, the standby signal STBY is set to H level indicative of "valid", while in a normal operation mode, the standby signal STBY is set to L level indicative of "invalid". If the switch is ON in the normal mode, i.e., if the standby signal STBY is L level, then oscillation by the crystal oscillator is performed. At this time, the crystal oscillator operates as a series resonance circuit having an intrinsic series resonance frequency f. The resisting means works as a feedback resistor. The NOR gate is operated as an inverting amplifier by the resisting means. If the series resonance frequency of the crystal oscillator is f, and the gain of the inverting amplifier is x1 or more, then the oscillation is maintained at the frequency f. In the low power consumption mode, i.e., when the standby signal STBY is H level, the NOR gate stops outputting the oscillation signal. The output terminal of the NOR gate is fixed at the ground potential.